1. Field of the Invention
The present invention relates to battery charge indicating devices, and particularly to battery charge indicators adaptable for mounting in a housing of a battery powered tool or appliance.
2. Description of the Prior Art
The development of electrical implements, such as portable power tools and small appliances, operated by rechargeable batteries has created a need for charge indicators for the batteries that power such tools and appliances. Such charge indicators must be durable, accurate, inexpensive, and adaptable to be mounted permanently in or on the housings of the battery operated tools and appliances. Another important requirement is that such charge indicators be rugged to withstand the rough handling usually received by tools and small appliances. The measuring circuit of the charge indicators must have low current drain to avoid significant discharge of the battery during measurement of the state of charge of the battery.
Portable battery charge indicating instruments are well-known for checking the state of charge of batteries by connecting a pair of leads of such instruments to the terminals of the batteries to be tested. Such instruments typically employ galvanometers with a circuit for loading the battery while the battery is being tested. These instruments are intended for occasional testing; they are inherently fragile and comparatively expensive.
Also known are digital voltmeters having liquid crystal numeric displays. Their analog to digital converters and complex digital displays make these voltmeters relatively expensive, and the displays must be large enough for the individual numbers to be easily readable.
Battery charge indicators have been permanently installed in storage cells to indicate the charge level as a function of changes in the specific gravity of the electrolyte. Such specific gravity indicators are used primarily with lead acid cells; they are unsuitable for batteries having a gel or solid electrolyte, such as the commonly used rechargeable nickel cadmium batteries.
The applicant's U.S. Pat. No. 3,820,875, which issued on June 28, 1974, discloses a scanner device having a field effect light scattering dielectric (e.g., a liquid crystal material) disposed between a pair of plates. The inner surface of one of the plates carries a resistor of predetermined length and configuration; the inner surface of the other plate carries a substantially nonresistive conductor, facing the resistor. At least one of the one plate and resistor and the other plate and conductor are transparent, so that the dielectric is visible through at least one of the plates.
A first voltage difference applied to the ends of the resistor produces a voltage gradient along the length of the resistor. Selective adjustment of a second, variable, voltage applied to the conductor changes the location on the one plate where the gradient voltage equals the conductor voltage to render the dielectric in that region visually distinguishable from the dielectric in the rest of the space between the plates.
In a three-plate embodiment, a first resistor extends along an edge of a first plate, and a second resistor extends perpendicularly to the first resistor along an adjacent edge of the plate. A series of parallel nonresistive conductor strips extend from each resistor perpendicularly across the opposite faces of the plate to form a grid, with parallel strips in one direction on one face of the plate and parallel strips in a perpendicular direction on the other face of the plate. The first plate is sandwiched between second and third plates, spaced from the respective faces of the first plate and each having a nonresistive conductor on the inner surface facing the respective series of parallel conductor strips. The spaces between the plates are filled with a field effect light scattering dielectric.
A voltage difference impressed on the ends of either resistor and an intermediate voltage applied to the facing conductor produce a-linearly extending visually distinguishable region in the dielectric between them at a location where the potential of a conductor strip equals the voltage applied to the facing conductor. If the dielectricis a liquid crystal, the visually distinguishable region normally is a transparent region. Since the transparent lines formed in the dielectrics on the opposite sides of the one plate are perpendicular to each other, only their intersection is transparent to light passing through both dielectrics. Thus a window is created at the intersection that appears as a visually distinguishable spot. When varying voltages are applied to the conductors on the second and third plates, the spot can be selectively scanned over the display.
U.S. Pat. No. 4,493,531 of BOHMER et al., issued on Jan. 15, 1985, introduces several alternative embodiments of the field sensitive optical display devices described above that can produce various patterns such as a movable dot, intersecting lines, and a pivoting line. Although the display devices of these patents are sturdy and inexpensive, they are not directly usable as battery charge indicators.